Move the Mac Task Manager to the new backend code.

chrome/browser/task_management/sampling/task_manager_io_thread_helper.cc

Index: chrome/browser/task_management/sampling/task_manager_io_thread_helper.cc
diff --git a/chrome/browser/task_management/sampling/task_manager_io_thread_helper.cc b/chrome/browser/task_management/sampling/task_manager_io_thread_helper.cc
index 67e8649c9cf448f9f43b184b0ebf1b9e2ee8a4d0..aadade4077120783d5a240347fa3f318b83b8aaf 100644
--- a/chrome/browser/task_management/sampling/task_manager_io_thread_helper.cc
+++ b/chrome/browser/task_management/sampling/task_manager_io_thread_helper.cc
@@ -26,7 +26,11 @@ IoThreadHelperManager::IoThreadHelperManager() {
 }
 
 IoThreadHelperManager::~IoThreadHelperManager() {
-  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+  // This may be called at exit time when the main thread is no longer
+  // registered as the UI thread.
+  DCHECK(
+      content::BrowserThread::CurrentlyOn(content::BrowserThread::UI) ||
+      !content::BrowserThread::IsMessageLoopValid(content::BrowserThread::UI));
 
   content::BrowserThread::PostTask(
       content::BrowserThread::IO,